Many digital video systems use conditional access (CA)in order to limit access to content consumption, for example, but not limited to, pay TV environment. CA generally requires the receiver to meet certain criteria before being able to decrypt and display a given content channel. The data stream transmitted for each channel is encrypted using a secret key, known as a control word (CW), which changes several times per minute. (This sort of encryption is commonly referred to as “scrambling.”) The CW itself is provided by entitlement control messages (ECMs), which are transmitted several times per minute from the system head-end to receivers of the channel. The CA subsystem (such as a secure smart card) in each receiver processes the ECM to recover the CW for a given content channel, but only when the receiver is authorized by an entitlement management message (EMM) for the given channel. EMMs are typically transmitted periodically (for example, once per month) to the CA subsystem, depending on the subscriber's service plan.
As digital television transmission has replaced analog broadcasts, viewers have found that the advantages of the newer mode are offset by a relatively long latency period when changing channels, as compared to the nearly instant response of a traditional analog tuner. This latency is largely a consequence of motion-compensated video encoding schemes, such as MPEG-2. When the transmitted content is encrypted, as is common in satellite and cable television systems, CA-related operations carried out by the receiver for each new channel can add further latency.
A number of attempts have been made to address this issue. For example, PCT International Publication WO 2010/079219 describes a method for delivery of conditional access (CA) data to a receiver that is able to connect to a plurality of scrambled programs. Conditional access for a current channel is handled using standard ECMs delivered with the channel. In parallel, the receiver retrieves an ECM for a separate stream comprising control words for every channel. The control word of this ECM is used to descramble the separate stream to access the control words for every channel. Upon channel change, the relevant retrieved control word is used to descramble the new channel. This approach is said to enable a reduction or removal of the CA delay when switching between channels.
As another example, U.S. Pat. No. 7,668,311 describes a method for transmission of decrypting information using a combination of authorization information for a channel (ECM), which is encrypted by a fast algorithm, with authorization information (MECM) for a group of channels, which is encrypted by a high-security algorithm. At the moment of change from one channel to another, the information contained in the mono-channel authorization message (ECM) suitable for the new channel is combined with the information contained in the multi-channel authorization message (MECM) already in the cryptographic unit, which is common to the two channels.
US Patent Application 2009/0285401 of Moroney, et al. describes conditional access using EMMs and ECMs.